Create abstraction barrier for _≤?_ from Data.Nat.Properties

[JacquesCarette]

As is, it makes goals really horrible.

[jamesmckinna]

Cf. #2174 ... from 900 issues/PRs ago, truly a golden oldie... oh wait, that's only 3 years old...

[JacquesCarette]

The true "golden oldie" is #31 from 12 years ago which is still open.

I have fond memories of trying (and failing!!!) to fix issue 0004 in Maple's old bug database (it has been redone multiple times, I don't know what the number is these days) from late 1980, where Maple itself had its first working version in December 1979. A bug in sum that turned out to be a bug in the theory. [I was doing archaeology at the time, as I think I was looking at that in 1993 or 94. Pretty sure I gave it another go later, once I understood the theory properly, but alas, still failed.]